A light-emitting diode device has various advantages such as low power consumption, high power, small-sized and light-weighted properties, etc. Therefore, it is expected to replace the position of conventional light sources such as electric light bulbs and fluorescent bulbs. Further, it is expected to be developed to new usages such as back lights for cell phones and LCD panels using its characteristic features. In order to increase the light emitting efficiency of the device, various proposals have been made, as described in non-patent literature 1 described later. For example, there have been proposed: a) to lower the temperature of the device by allowing heat to escape from the light-emitting diode by a heat radiating plate; b) to take out the light emitted from the light emitting diode by a reflecting plate; and c) to lower the temperature of the device by allowing heat to escape from the light-emitting diode by using a material such as metal which has high heat conductivity, for printed circuit boards and boxes.
FIGS. 1 and 2 each shows a cross sectional view illustrating the use of an adhesive in a conventional diode device. In FIG. 1, a light-emitting diode device 1 has a fluorescent material-containing resin 7 placed on a light-emitting diode element 5 which is connected to a circuit board 3, and the resin 7 is covered by a lens 8. A reflecting plate 4 is positioned around the lens 8 to increase light efficiency. On the side opposite to the light-emitting diode element 5 relative to the circuit board 3, there is provided a heat-radiating plate or box 2. In FIG. 1, an adhesive 6 is used to bond the heat-radiating plate or box 2 to the circuit board 3, or to bond the reflecting plate 4 to the circuit board 3. In FIG. 2, a different type of a light emitting diode device 1 is shown in which a fluorescent material-containing resin sealing frame 9 is used in place of the reflecting plate. In FIG. 2, an adhesive is used to bond the frame 9 to a circuit board 3.
The light-emitting diode device is produced generally by mounting a light-emitting diode element 5 to a circuit board 3, and adhering thereto a reflecting plate 4, a heat-radiating plate or box 2 or a fluorescent material-containing sealing frame 9, etc. Subsequently, a liquid resin is filled to seal the phosphor and the light-emitting diode element 5 and/or to fix a lens 8, and heated to solidify them.
The light-emitting diode device should have a high brightness and long operating life. When the brightness level is raised, there increases the heat from the light-emitting diode element and the adhesive used should have a high, continuous heat-resistance. Because the light-emitting diode device is thought to be used outdoor, it should have a long operating life and a weather resistance and, accordingly, the adhesive used should have a high continuous heat-resistance and weather resistance.
In conventional light-emitting diode devices, there have been used adhesives such as liquid type thermo-setting adhesives as described in Patent publication 1 and sheet-type thermoplastic adhesives as described in Patent publication 2.
When the liquid type thermo-setting adhesive is used, a large amount of the adhesive gets out from the adhering portion at an adhering process, and adheres to the light-emitting element which causes a reduction in the light-emitting efficiency of the device. Further, because much gas is generated, chemical substances are adhered to the light-emitting diode element and the light-emitting efficiency of the device is damaged. The sheet-type thermoplastic adhesive is poor in heat resistance. Therefore, when such adhesive is used it cannot endure the heat and stress applied thereto in the sealing process of the light-emitting diode element, and the molding resin gets through adhering parts. Further, there arises a problem that the adhesive cannot endure the heat applied during the operation of the light-emitting diode device, and it may be peeled off.
In Patent publication 3, there is described a sheet-type thermo-setting adhesive of an epoxy resin or polyimide resin material, in addition to said liquid type epoxy resin or polyimide resin-based adhesives. Although there is no concrete description about what type of the epoxy resin or polyimide resin was used, it is not possible to obtain sufficient adhesive power when a normal epoxy resin is used in a small adhering area. Further, because an epoxy resin has a high elastic modulus (108 Pa or more) after hardening of the resin, there is a problem that it cannot endure heat and stress produced and applied to the resin at the sealing process of the light-emitting diode element, resulting in cracks and peeling off of the adhered portions. in addition, there occur cracks and peel-off at the adhered portion by the stress due to the difference in the linear expansion coefficients between circuit boards and the heat radiating plate or reflecting plate, caused by the heat generated from the light-emitting diode element or the heat generated at the solder re-flowing process.
The above-mentioned liquid type thermo-setting adhesives and sheet-type thermoplastic adhesives of a general epoxy resin base contain a high amount of low molecular weight components in their resin compositions. Accordingly, much amount of gas is generated from the adhesives by the heat produced in the production or provision process of the light-emitting diode devices and by the heat applied during the operation of the devices. This is a great problem because the generated gas adheres to the light-emitting diode element, causing a decrease in the light-emitting efficiency.
[Non-patent publication 1] Application and Future Prospect of White LED illumination System Technology (CMC Publishing)
[Patent publication 1] Patent Publication No. 2,948,412
[Patent publication 2] Kokai (Jpn. Unexamined Patent Publication) No. 2000-353828
[Patent publication 3]Kokai (Jpn. Unexamined Patent Publication) No. 2001-144333.